Electromagnetic induction heating, referred to as induction heating, is a method of heating conductor materials such as metal materials. Mainly used for metal hot working, heat treatment, welding and melting.
As the name implies, induction heating uses electromagnetic induction to generate current in the heated material, and relies on the energy of these eddy currents to achieve the purpose of heating. The basic components of induction heating system include induction coil, AC power supply and workpiece. The coil can be made into different shapes according to different heating objects. The coil is connected with a power supply, which provides alternating current for the coil, and the alternating current flowing through the coil generates an alternating magnetic field to pass through the workpiece, so that the workpiece generates eddy current heating.
Induction heating principle
Induction heating surface quenching is a quenching method that uses electromagnetic induction principle to generate high-density induction current on the surface of workpiece, quickly heats it to austenite state, and then quickly cools it to obtain martensite structure. When alternating current with a certain frequency passes through the induction coil, an alternating magnetic field with the same frequency as the current will be generated inside and outside it. When a metal workpiece is put into an induction coil, under the action of a magnetic field, an induction current with the same frequency and opposite direction will be generated in the workpiece. Because the induced current forms a closed loop along the surface of the workpiece, it is usually called eddy current. This eddy current converts electric energy into heat energy, which quickly heats the surface of the workpiece. Eddy current is mainly distributed on the surface of the workpiece, and almost no current passes through the workpiece. This phenomenon is called surface effect or skin effect. Induction heating is to quickly heat the surface of workpiece to quenching temperature by using skin effect and relying on the thermal effect of current. The induction coil is made of copper tube and filled with cooling water. When the surface of the workpiece is heated to a certain temperature in the induction coil, it is immediately cooled by spraying water, so that the surface layer can obtain martensite structure.
The instantaneous value of induced electromotive force is:
Where: e-instantaneous potential, v; φ —— the total magnetic flux of the area surrounded by the induced current loop on the part, Wb. Its value increases with the increase of the current intensity in the inductor and the magnetic permeability of the part material, and it is related to the gap between the part and the inductor.
Is the rate of change of magnetic flux, and its absolute value is equal to the induced potential. The higher the current frequency, the greater the change rate of magnetic flux, and correspondingly the greater the induced potential p. The negative sign in the formula indicates that the direction of induced potential is opposite to the direction of change.
At every moment, the direction of the eddy current induced in the part is opposite to the direction of the current in the inductor, and the intensity of the eddy current depends on the induced potential in the part and the reactance of the eddy current loop, which can be expressed as:
Where I- eddy current intensity, a; Z- self-inductance reactance, ω; R- device resistance, ω; X impedance, ω.
Because z value is very small, I value is very large.
The heat of the heating part is:
Where q- heat energy, j; T- heating time, s.
For ferromagnetic materials (such as steel), the thermal effect produced by eddy current heating can quickly increase the temperature of parts. Iron and steel parts are hard magnetic materials with great remanence. In the alternating magnetic field, the magnetic pole direction of the part changes with the change of the magnetic field direction of the inductor. Under the action of alternating magnetic field, magnetic molecules will produce strong friction and heat due to the rapid change of magnetic field direction, and also play a certain role in heating parts, which is hysteresis thermal effect. This part of heat is far less than the thermal effect of eddy current heating. The hysteresis thermal effect of steel only exists below the magnetic transition point A2(768℃), and the steel above A2 loses its magnetism. Therefore, for steel parts, the heating speed below A2 is faster than that above A2.
Specific application of induction heating
Induction heating equipment
Induction heating equipment is the equipment that generates induction current with specific frequency, carries out induction heating and surface quenching treatment.
Induction heating surface quenching
Put the workpiece into an inductor wound with a hollow copper tube, and after applying an intermediate-frequency or high-frequency alternating current, an induced current with the same frequency is formed on the surface of the workpiece, so that the surface of the workpiece is rapidly heated (the temperature can rise to 800 ~ 1000 degrees in a few seconds, and the core is still close to room temperature), and then the surface of the workpiece is immediately cooled by water spraying (or quenched by oil immersion).
Compared with ordinary heating quenching, induction heating surface quenching has the following advantages:
1, extremely fast heating speed can expand the transition temperature range of an object and shorten the transition time.
2. After quenching, fine cryptocrystalline martensite with slightly higher hardness (2 ~ 3 hr c) can be obtained on the surface of the workpiece. Low brittleness and high fatigue strength.
3. The workpieces treated by this process are not easy to be oxidized and decarburized, and even some workpieces can be directly assembled and used after treatment.
4. The hardened layer is deep, easy to control and operate, and easy to realize mechanization and automation.
Induction heating (high frequency electric furnace) production course
Cost estimation:
Copper tube and copper strip: 2 10 yuan
Two EE85 thick cores: 60 yuan.
3 High frequency resonance capacitance: 135 yuan.
Bakelite board: 60 yuan
Water pump and PU pipe: 52 yuan
PLL board: 30 yuan
GDT board: 20 yuan
Power board: 50 yuan
MOSFET: 20 yuan
2KW voltage regulator: 280 yuan
Radiator: 80 yuan
* * * meter: 997 yuan
Overall architecture:
Series resonant 2.5KW phase-locked loop chasing ZVS, MOSFET full-bridge inverter;
Two-step impedance transformation of magnetic core transformer, water cooling and heat dissipation, self-coupling voltage regulation and power adjustment of commercial power, and bus overcurrent protection.
Preview the effect first, as shown below:
Heating gold sealing pipe 3DD 15
Heat 304 stainless steel pipe
Heating small metal balls
Heating ironing machine
Before starting production, it is necessary to clarify some basic principles and concepts to avoid confusion.
1, heating mechanism (for literacy, experts skip)
1. 1 eddy current, as long as the metal object is in an alternating magnetic field, it will produce eddy current, and the powerful high-density eddy current can quickly heat the workpiece. This mechanism exists in all conductors whose resistivity is not infinite.
1.2 induction cycle, the workpiece is equivalent to a short-circuit coil of 1 turn, and forms an air-core transformer with the induction coil. Because the current ratio is equal to the inverse ratio of turns, the current on the workpiece is n (turns) times that in the induction coil, and the strong induction short-circuit current makes the workpiece heat up rapidly. This mechanism exists in any conductor. Under the condition of constant magnetic flux density, the larger the area of the workpiece orthogonal to the magnetic field vector, the greater the current induced on the workpiece and the higher the efficiency. It can be seen that the workpiece with large magnetic flux cutting area is easier to obtain high temperature than the workpiece with small area.
1.3 magnetic domain friction (there are countless small magnetized regions with linearity of about 10-4m in ferromagnetic body, which are called magnetic domains), and the magnetic domains of ferromagnetic materials are violently rubbed under the magnetization of alternating magnetic field and the action of anti-magnetic ring, resulting in high temperature. This mechanism is dominant in ferromagnetic materials.
It can be seen that the heating effect of different materials is different because the heating mechanism is different. Among them, ferromagnetic substances account for all of the three mechanisms, and the heating effect is the best. When ferromagnetic material is heated above Curie point, it becomes paramagnetic, and the magnetic domain mechanism weakens or even disappears. At this time, heating can only be continued through the remaining two mechanisms.
When the workpiece crosses the Curie point, the magnetic induction phenomenon is weakened, the equivalent impedance of the coil is greatly reduced, and the current of the resonant circuit is increased. After crossing the Curie point, the inductance of the coil also decreases. The natural resonant frequency of the LC loop will change. The heater with fixed excitation mode is out of tune, which leads to equipment damage or greatly reduced efficiency.
2. Why use resonance? What kind of resonance should I use?
2. 1 Answer the first question first. I used to think that as long as a strong enough current was injected into the induction coil, it was an induction heating device. We also did an experiment on this, as shown in the figure below.
There is indeed a heating effect in the experiment, but it is far from the effect of power output. Why is this? Let's analyze it. Obviously, for a fixed workpiece, the thermal effect is directly proportional to the actual output power of the inverter. For the induction coil, it is basically pure inductance, that is, the current change at both ends always lags behind the voltage change, that is to say, when the voltage reaches the peak, the current has not reached the peak, and the power factor is very low. We know that power is equal to the overlapping area of voltage waveform and current waveform, but in inductance, current and voltage waveforms are staggered by an angle, and the overlapping area is very small at this time, even if a huge current passes through, it is useless. That is, if you simply calculate P=UI, all you get is reactive power.
For capacitors, on the contrary, the current between them is always ahead of the voltage change. If the capacitor and the inductor resonate in series or parallel, one is in front and the other is in the back, and the resonance just cancels out. Therefore, capacitors are also called power compensation capacitors here. At this time, from the point of view of excitation source, it is equivalent to pure resistive load power supply, and the current waveform and voltage waveform are completely coincident, and the maximum active power is output. This is the main reason why series (parallel) compensation capacitors are used to form resonance.
2.2 second question, LC resonance has series resonance and parallel resonance, which structure should be adopted?
To put it bluntly, in the parallel resonant circuit, the resonant voltage is equal to the excitation source voltage, and the current in the tank circuit is equal to q times the excitation current. The tank current of the series resonant circuit is equal to the excitation source current, while the voltages across L and C are equal to Q times the excitation source voltage, each of which has its own advantages and disadvantages.
From the circuit structure:
For constant voltage source excitation (half bridge, full bridge), series resonant circuit should be adopted, because the power supply voltage is constant, the greater the current, the greater the output power. For the series resonant circuit, the impedance of the whole circuit is the smallest at the resonant point, the resonant current also reaches the maximum, and the maximum power is output. When in series resonance, the Q value of the no-load loop is the highest, and the voltage at both ends of L and C is high, so the tank current is wasted on the loop resistance, resulting in huge heat.
For constant current source excitation (such as single-tube circuit), parallel resonance should be adopted. The terminal voltage of LC is very high at free resonance, so it can obtain great power. Parallel resonance has a very important advantage, that is, the loop current is the smallest and the heating power is also very small when there is no load. It is worth mentioning that from the experimental results, the same resonant capacitor and heating coil, the same driving power, parallel resonance is suitable for heating larger workpieces, and series resonance is suitable for heating smaller workpieces.
3. Production process
After understanding the above principles, we can start to build our induction heating equipment. The equipment we made is mainly composed of voltage-regulating rectifier power supply, phase-locked loop, dead-time generator, GDT circuit, MOS bridge, impedance transformation transformer, LC energy storage circuit and heat dissipation system, as shown in the following figure.
Let's analyze the schematic diagram of the system again, as follows:
Slot part:
As can be seen from the above figure, C 1, C2, C3, L 1 and the secondary (left) * * of T 1 are isomorphic to form a series resonant circuit. Because there is leakage inductance in the transformer secondary and distributed inductance in the circuit wiring, the actual resonance frequency is higher than L65438 only with the capacity of C 1-C3. In the figure, L 1 is actually 1uH, and I add leakage inductance and distributed inductance, so it is1.3uh. As shown in the figure, the parametric resonance frequency is 56.5KHz.
The high-frequency square wave excitation signal output by the inverter bridge is input from J2- 1, passes through DC blocking capacitor C4 and single-pole double-throw switch S 1, enters the primary of T 1, then flows through 1: 100 current transformer and flows back to the inverter bridge from J2-2. Here C4 is simply used as a DC blocking capacitor and does not participate in resonance, so it is necessary to choose a non-inductive nonpolar capacitor, and the capacitor is large enough. Here, five CDE non-inductive absorption capacitors 1.7uF 400V are selected in parallel to reduce heating.
S 1 is used to switch the impedance transformation ratio. When the switch hits the upper contact, the turns ratio of the transformer is 35:0.75, and the converted impedance transformation ratio is 2178:1; When the switch hits the lower contact, the turns ratio of the transformer is 24:0.75, and the converted impedance ratio is 1024: 1. Why to set this impedance ratio switching is mainly based on the following reasons. The size of (1) ferromagnetic workpiece determines the equivalent resistance of the whole series resonant circuit. The larger the size, the greater the equivalent resistance. (2) There is a huge difference in equivalent resistance between no-load and load circuits. If the no-load time-varying ratio is too low, the inverter bridge will burn out instantly.
T2 is a sampling transformer with T 1 primary working current. Because the turns ratio is 1: 100 and the load resistance is100Ω, when the voltage on the resistor is 1V, the primary current of T 1 corresponds to1a. Transformer leakage inductance should be small, easy to manufacture, and ferrite magnetic slots should be used. If there is no magnetic tank, a magnetic ring can be used instead. When debugging the circuit, the oscilloscope can be used to detect the waveform and amplitude of the voltage across the JBOY3, to know the working state, frequency, current and other parameters of the circuit, and can also be used as the sampling point for overcurrent protection.
Terminal J 1 outputs the voltage signal across the resonant capacitor. When the circuit resonates, there is a 90 phase difference between the capacitor voltage and the secondary voltage of T 1. When the signal is sent to the subsequent PLL, the excitation frequency can be automatically adjusted and always equal to the resonance frequency. And the phase is constant. (detailed later)
The coils of L 1 and T 1 are all copper tubes, and the data are shown in the above figure. During the operation, the coil is seriously heated, so water cooling measures must be added to ensure long-term safe operation. In order to ensure good transmission characteristics and prevent magnetic saturation, T 1 uses two EE85 cores to overlap. When winding the coil, it is necessary to make a mold with a cross section slightly larger than that of the tongue of the magnetic core with wooden board, wind it around it and then demould it. As shown in the figure below:
PLL PLL part:
The above picture is the PLL part, which is the core of the whole circuit. The structure and working principle of CD4046 chip will not be described in detail here. Please consult books or the Internet yourself.
The chopper-stabilized current switching circuit with U 1 five-terminal single-chip switching power supply chip LM2576-adj as the core provides a stable and powerful power supply for the whole PLL board. The parameters in the figure can provide a stable voltage of 15V2A. Because of the power supply of 15V VDD, the chip can only use CMOS devices of CD40xx series, and 74 series cannot work at this voltage.
The VCO oscillation signal inside the CD4046 PLL chip is output from four pins. On the one hand, it is sent to the dead-time generator with U2 as the core to drive the subsequent circuits. On the other hand, it is fed back to pin 3 of phase detector input B port of CD4046. The frequency range of on-chip VCO is determined by the values of R 16, R 16, W 1 and C 13. As shown in the figure, with the change of VCO control voltage 0- 15V, the oscillation frequency changes between 20KHz-80KHz.
The voltage signal from the Vcap interface J 1 of the resonant circuit is input to the PLL board from the J4 interface, and after passing through the clamping circuit composed of R 14, D2 and D3, it is sent to the 14 pin of the input port of the CD4046 phase detector. It should be noted here that the phase of the Vcap voltage needs to be reversed to form negative feedback. For D2 and D3, it is recommended to use detector tubes or switch tubes with low junction capacitance, such as 1N4 148, 1N60.
C7 and C 12 decouple the power supply of CD4046 and bypass the high-frequency components in the power supply to make it work stably.
Now let's talk about the workflow. We choose the phase detector 1(XOR exclusive OR gate) in CD4046. For the phase detector 1, when the level states of the two input signals Ui and Uo are different (that is, one is high level and the other is low level), the output signal u ψ is high level; Conversely, when the Ui and Uo level states are the same (that is, both are high level or low level), the U ψ output is low level. When the phase difference Δ φ between Ui and Uo changes in the range of 0- 180, the pulse width m of U ψ also changes, that is, the duty cycle also changes. From the input and output signal waveforms of comparator I (as shown in Figure 4), it can be known that the frequency of its output signal is equal to twice that of the input signal, and it keeps a 90-degree phase shift with the central frequency between the two input signals. It can also be seen from the figure that fout is not necessarily a symmetrical waveform. For phase comparator I, it requires the duty cycle of Ui and Uo to be 50% (square wave) to maximize the locking range. As shown below.
As can be seen from the above figure, when the phase difference between pin 14 and pin 3 changes, the output pulse width of pin 2 also changes, and the PWM signal of pin 2 passes through an active low-pass filter with U4 as the core to obtain a relatively smooth DC level. Taking this DC level as the control voltage of VCO can form negative feedback, locking the output signal of VCO and the input signal of 14 pin at the same frequency with fixed phase difference.
As for the dead-time generator, this circuit consists of U2 CD400 1 42 input NAND gate and peripheral R8, R8, C 10, C1* *. Using the delay time of RC charging and discharging, the real-time signal and the delayed signal are AND-operated, and the appropriate dead time is obtained. The time of death is determined by R8, R8, C 10, C 1 1 * *. As shown in the figure, the parameter is about1.6us. In actual design and installation, the ceramic capacitor of 68pF and the adjustable capacitor of 5-45pF should be used in parallel for C 10 or C1to adjust the dead-time symmetry of two groups of driving waveforms.
The following figure clearly shows the effect of the dead zone.
As for the totem output, the level signal output by the dead-time generator has only a weak driving ability, and its output power must be amplified to a certain extent, so as to effectively promote the subsequent GDT (Gate Driven Transformer) part. Q 1-Q8 constitutes a bipolar emitter follower, commonly known as totem pole, which converts high input impedance into extremely low output impedance and is suitable for driving power loads. R 10。 R 1 1 is a pull-up resistor, which enhances the strength of the "1" output of CD400 1. Some people may ask whether it is unnecessary to design a two-stage totem. I thought so at first. During the test, only the first-level TIP4 1 and TIP42 are used as totem outputs. After the test, it is found that the inclined drop of high pressure flat roof is more serious after loading. After analysis, the hFE of this type of transistor is too low, and the flat top slope disappears after adding the previous stage 8050/8550.
GDT gate drive circuit;
The above picture shows the gate drive circuit of MOSFET. The advantage of using GDT driver is that even if there is a problem with the driver stage, it is impossible to have a * * * state conduction excitation level.
Leave a proper dead time, the dead time of this circuit is as high as 1.6uS, and the MOSFET switches quickly, without the tail of IGBT, it is difficult to blow up the tube. The Miller effect of MOS is much smaller.
The circuit is in ZVS state, and the lamp basically does not generate heat when it works at 2KW, so the thermal breakdown no longer exists.
Two inverted driving signals output by totem pole of phase-locked loop board are respectively input from J 1 and J4 interface of GDT board, isolated by c 1-C4 DC and sent to pulse isolation transformer T 1-T4. The existence of R5 and R6 reduces the DC blocking capacitor and the oscillation Q value of the primary side of the transformer, which plays a role in reducing overshoot and ringing. The floating pulse of 15V output by the pulse transformer passes through the current-limiting buffer of R 1-R4 (which prolongs the charging time of Cgs and slows down the conduction slope), and then is clamped bidirectionally by Zener diode ZD 1-ZD8, and finally is output to the gs poles of four MOS transistors through J2, J3, J5 and J6 terminals. Here, because the turn-off period is-15V, even if there is a little level jitter, the MOS tube will not be turned on abnormally, resulting in * * * state conduction. Note that J2 and JBOY3 are used to drive one diagonal mos transistor, and J5 and J6 are used to drive another diagonal MOS transistor.
In order to effectively use the power output by the totem of PLL board and reduce the height of the driving board, four pulse transformers are used to drive four lamps respectively. The pulse transformer T 1-T4 adopts the core of EE 19 without air gap, and the primary and secondary windings are all wound with 30T enameled wire of 0.33mm. In order to improve the withstand voltage between windings, parallel windings do not use double wires. Instead, it is wound on the primary winding, insulated with three layers of high-temperature resistant tape, and then wound on the secondary winding, using dense winding method, paying attention to the homonym ends indicated by+and-in the figure. CBB nonpolar capacitor is used for C 1-C4. The rest is based on circuit parameters.
Power supply part:
The above picture shows the bus power supply part. The mains voltage is input from J2 after passing through autotransformer, rectified by B 1 and sent to C 1-C4 for filtering. In order to keep the bus voltage constant (constant voltage source) during MOS bridge switching, the filter inductance is not increased. C 1, C2 is MKP capacitor, which is mainly used for reverse surge absorption during full-bridge clamping. The pulsating DC after rectification and filtering is output from J 1
Full bridge part:
The above picture shows the MOSFET bridge circuit, which has a simple structure and will not be described in detail. It is emphasized that the lead wire between GS electrode and GDT plate of each MOS tube should be as long as possible, but less than 10cm. Twisted pair must be used. The selection of MOS transistor should follow the following requirements: switching time is less than 100nS, withstand voltage is higher than 500V, internal damping diode is provided, current is greater than 20A, and dissipation power is greater than150 W. ..
4. Cooling system
At full power output, the current flowing in the secondary of the impedance transformation transformer and the induction coil in the energy storage circuit reaches 500A, and it will overheat and burn out in a short time without strong cooling measures.
The system should adopt water cooling measures and use the copper tube itself as the water flow path. The pump adopts diaphragm pump, which is self-priming high pressure. In the circuit, the domestic Prandy diaphragm pump is used, and the output pressure reaches 0.6MPa, so it is easy to realize large flow water cooling in a 3mm inner diameter copper tube.
Step 5 assemble
Assemble according to the following figure, and pay attention to the GDT part. Output port 1 pin G, 2 pin S, twisted pair length less than 10cm.
Step 6 troubleshoot
The debugging of this circuit is relatively simple, mainly divided into the following steps.
The overall function detection of 1.PLL board. After the circuit is assembled, the high-voltage power supply is disconnected first, and the pins 2 and 3 of the jumper JP 1 of the PLL board are short-circuited, so that the VCO outputs a square wave with a fixed frequency. Then use oscilloscope to detect the GS voltage of four MOS tubes to see if it meets the requirements of phase and amplitude. Diagonal waveforms are in phase, and waveforms of the same arm are in phase. The amplitude is 15V. If there is no problem in this step, proceed to the next step. If the waveform phase is abnormal, detect whether the twisted pair connection is wrong.
2. Adjustment of dead time symmetry. Monitor the GS voltage of two MOS in the same arm with an oscilloscope, and adjust the adjustable capacitance of PLL board C 10 or C 1 1 in parallel, so that the high-level width of GS voltage of two MOS is basically the same. If the difference of dead time is too large, it is easy to cause the accumulated bias of the magnetic core and the explosion of the saturated tube in the first few cycles of oscillation, and DC blocking capacitor can alleviate this situation.
3.VCO center frequency adjustment. In PLL loop, when the center frequency of VCO is near the resonance frequency, the maximum tracking and capture range can be obtained, so it needs to be adjusted. When the slot S 1 is switched to the upper contact, the pins 2 and 3 of the jumper JP 1 of the PLL board are short-circuited, so that the VCO control voltage is 0.5VCC and W2 is placed at the midpoint. The high voltage input is regulated to 30VAC by autotransformer. Monitor the high-voltage input current with multimeter AC current file, monitor the J3 interface voltage of tank circuit with oscilloscope, and slowly adjust the W 1 of PLL board to make the J3 voltage a standard sine wave. At this time, the reading of the ammeter is also the maximum. At this time, the resonance frequency is basically equal to the center frequency of VCO.
The waveform at resonance is as follows. The current waveform is a standard sine wave, which lags behind the driving waveform by about 200nS.
4.PLL lock adjustment. Short-circuit the 1 and 2 pins of the PLL board JP 1 jumper, so that the voltage control right of VCO is transferred to the phase discrimination filter network. Keep the high voltage input at 30VAC, and monitor the waveform and frequency of JBOY3 interface voltage in the energy storage circuit with an oscilloscope. At this time, adjust W 1 by one turn with a screwdriver. If the oscilloscope waveform frequency remains the same, the shape is still a good sine wave. Mean that that circuit is almost stably locked. If it cannot be locked, switch the wiring of J 1 of the energy storage circuit and repeat the above steps. When you see that the circuit is locked, put the screwdriver into the heating coil. At this time, due to the large equivalent load impedance, the waveform amplitude decreases, but it still maintains a good sine wave. If the lock is lost at this time, fine-tune W 1 to keep the lock.
5. Current lag angle adjustment. After the circuit is locked, monitor the JBOY3 interface voltage of the energy storage circuit and the GDT2 or GDT 1 interface voltage of the PLL board with an oscilloscope, and slowly adjust W2 so that the current waveform (sine wave) lags behind the driving voltage waveform slightly. At this time, the load of the whole bridge is weakly inductive and enters the ZVS state.
6. Workpiece heating test. After all the above steps are successful, the workpiece can be heated. First, put the workpiece in, and monitor the high voltage current with the multimeter current file. Slowly increase the output voltage of autotransformer, and you can see that the workpiece begins to heat up. Ensure that the current is less than 15A at a high voltage of 220VAC. At this time, when the power reaches 2500W W, when the workpiece with large volume is heated, it is necessary to switch the slot section S 1 to the lower contact due to the large equivalent impedance.
At this point, the whole induction heating circuit has been debugged. Start feeling the high temperature experience.